I'd like to start a little discussion about battery impedance (mainly resistance) and how it affects the performance of a battery, particularly its thermal performance.

A couple of basics - when a battery is incorporated into a load circuit, there are several sources of resistance. The copper conductors, the load itself, the terminals and their surfaces. But there is also a resistance inside the battery - internal resistance, normally measured as a few milliohms. Tiny fractions of an ohm seem negligible until you start pushing very high currents. The relationships at play are V=IR and P=VI. It also follows from these equations that P=I^2 R. That is, for a given resistance, the power loss will increase by the square of the current flowing through it. The thermal conductivity of metals matters also, because the two are related. As the temperature of a metal increases, its conductivity decreases. The reverse is true too - superconductors do their thing at -170'C.

Now I'm raising this because in my current job, and many projects before, minimising weight is a very popular activity. One part of an EV which often attracts attention is the conductors - particularly the series conductors within a battery. These are invariably made of copper and are tin or nickel plated for corrosion resistance. Occasionally, they are made from aluminium in an effort to reduce weight.

Copper is the most conductive metal at room temperature, bettered only by silver (about 10% better). Aluminium is fourth behind gold at about half that of copper. Aluminium is about a third of the density of copper though, so if you make the conductor thick enough it can still be lighter than copper and do the same job (though it tends to form oxide layers, which may in turn decrease the conductivity).

I have made an observation concerning battery terminations which I think is explained nicely with some LiPo cells I soldered together. Typically these cells can deliver hundreds of amps at a mere 3.7 volts (that is, they are very low internal resistance cells). However when a 0.4 ohm load (galvanised wire) was applied to the cells which were soldered tab-to-tab, the cells got VERY hot, especially around the tab and the top of the cell. The conductor was effectively 0.2 mm thick and 10 mm wide, pushing through a Pb/Sn soldered joint.

When the same battery was built with 2 mm thick, 15 mm wide copper conductors, the cells dished out more amps, and did not get hot until the final stages of the discharge curve. Soldered or screwed, it still managed to dish out epic current without getting hot.

Now the copper added weight, but I am usually reluctant to skimp on this part of the system for the reasons demonstrated above. Any heat being generated at the tab is being soaked up by the cell, causing it to overheat. If there is a lump of copper fixed to it, the copper can soak up that heat, leaving the cell to keep on trucking.

Now this all seems rather obvious: P=I^2 R and R increases with temperature. But I am seeing lots of builds using 18650 cells which have thin nickel strips spotwelded together. Nickel is about 23% as conductive as copper, and thin strips only make the heating worse.

Even Tesla uses a relatively thick (what looks to be) nickel busplate on their modules. They are liquid cooled, but there is only so much heat a glycol loop can take out through a layer of kapton for galvanic isolation. The Tesla cells are famously fused with individual fusewire linking the terminal to the busplate, but as we know with fuses and my example above, any heat generated is quickly soaked away by the adjacent metal.

My long-winded and rambling rhetorical question is, are the cells in these 18650 cell packs getting hot (and needing some kind of active thermal management) because the builders skimped on the conductors?

All true.
"Hot" is relative over time.
Panasonic suggest a discharge temperature between -20DegC to +60DegC.

It is very plausible that bike builders after some use the pack resistance deteriorates which can cause hot spots within the pack.
That is the risk they take if they push the batteries hard and have no temp sensor.
Even el-cheapo bunnyking drill batteries have a temp sensor in them.

In a regular car ev the cells aren't pushed hard so this topic doesn't come up much.
It is possible to measure the resistance on the fly and infer temp without a temp sensor.
But it becomes tricky due to thermal mass and external cooling.

TBH you can get away with one or two sensors in the pack have have it on a display just like a regular temp gauge.
The thermal management is then your foot...

On a side note why haven't those sodding battery people taken the initiative and put end caps on the 18650 like the headway cells?
I can already, and have, buy everything else for them.
Just imagine how easy it would be to unbolt a dead cell out of a racebike, pop it out and slide in a new one...

Help prevent road rage - get outta my way! Blasphemy is a swear word. Magnetic North is a south Pole.

An interesting observation about batteries is that the more robust you make the battery, the harder it is to repair. But by building it with repairability in mind, you are introducing new failure modes (loosening contacts through vibration and fretting etc). So I guess they've got to a point where the quality control is so good that the likelihood of a dud cell is so low, you can rest easy knowing it's not going to fail.

And I think many of the heat issues around 18650 cells stem from the fact they were never made with high discharge in mind. The format was chosen in the late 1990s for chunky laptops, where a 1 amp load was huge. There was no thought given to the thermal properties of a stainless steel can, or nickel strips spot welded to the tabs taking any more than an amp. Along came Tesla and all of a sudden, some complicated cooling systems are being created to manage heat in a cell that was never designed with high power from the get-go.

***

Active cooling takes up valuable real estate and mass in an EV. Cooling (or heating) plates between each cell can start to add up, so I am especially impressed with the likes of the Renault Zoe or Chevy Bolt with 41 and 60 kWh batteries respectively. They have some very detailed systems in place to keep the cells at their happy temperature all year round, 24/7.

With bike racing we are not too concerned about heat as the races are short and we have an hour between heats. But we also don't care too much about cycle life - in 2015 we did 6 rounds, effectively 8 races per round, Friday practice and qualifying... so call it 50 or 60 cycles a year. The calendar life will beat it before the cycle life does.

But if we were selling a battery to be used in regular daily activities, where the power demands were low and cycles were daily, thermal management is the key to battery longevity. Jeff Dahn's presentations illustrate quite clearly the effect of temperature and state of charge on the battery's expected lifespan, and the rate at which coulombic efficiency drops. Tesla knows this, and that's why they have a thermal management loop maintaining the cells to between 10'C and 30'C even when the vehicle is not in use.

Anyone have hard data on how much internal impedance increase on their cells against cycle life (or maybe in the form of voltage droop)?

My e-Bike battery has passed 1000 cycles (1037 at the moment) and for the first time ever in the middle of stormy ride yesterday my BMS cut off at the 75% (8.7Ah out of 11.6 Ah capacity). The cell voltage only dropped to 2.9V (but that's average, the actual instant drop would be less).

jonescg wrote:And I think many of the heat issues around 18650 cells stem from the fact they were never made with high discharge in mind. The format was chosen in the late 1990s for chunky laptops, where a 1 amp load was huge. There was no thought given to the thermal properties of a stainless steel can, or nickel strips spot welded to the tabs taking any more than an amp. Along came Tesla and all of a sudden, some complicated cooling systems are being created to manage heat in a cell that was never designed with high power from the get-go.

And yet, when they decided to build a factory to build their own cells, and so could have built anything, they decided to build something just like an 18650 only 10% bigger in all dimensions -- the 20700 cell.

One of the fathers of MeXy the electric MX-5, along with Coulomb and Newton (Jeff Owen).

There is some merit in the theory that a smaller cell means manufacturing defections are easier to identify at the QC stage than in a larger cell. I'm guessing they have weighed it up against the added cost of a high part count and increased automation to assemble complicated packs, and the numbers worked out better.

I did find a paper which explores the resistance path through a pouch cell tab. Will link it here when I get back to work on Monday, but it suggested up to 10% of the impedance was due to the size of the tab.

You can see in those graphs, the Arrhenius-law aging with temperature -- approx doubling of the aging rate for every 10 °C rise -- no surprises there. But you can also see that LFP barely cares about the difference between 80% and 100% SoC, but with NCA and NMC it's chalk and cheese, particularly for internal resistance increase. Presumably it's the 4.2 volts that does the damage.

One of the fathers of MeXy the electric MX-5, along with Coulomb and Newton (Jeff Owen).

jonescg wrote:My long-winded and rambling rhetorical question is, are the cells in these 18650 cell packs getting hot (and needing some kind of active thermal management) because the builders skimped on the conductors?

Hi Chris, Don't you hate it when you say your question is rhetorical and some bastard insists on answering it? Well I'm not quite going to do that.

The links in my previous posts certainly support the idea that, so long as you're not letting your cells spend significant time near 4.2 volts, and so long as you're not cycling them deeply every day, then temperature is the number one killer, and there's an exponential relationship between aging and temperature. So they totally support your thesis that it's crazy to skimp on the size of your cell interconnects. Folks can google "busbar current" without the quotes, and look at the temperature rises. You really don't want more than a 10 °C rise for your cell interconnects at max continuous current.

I suspect that what you describe as "nickel" strips are really copper strips with a very thin nickel coating for looks and corrosion resistance. The nickel coating would be so thin it would have no impact on resistance -- thermal or electrical.

Here's something people should know about copper oxide: Unlike aluminium oxide which is like glass -- transparent and a serious insulator, copper oxide is a semiconductor, with somewhat variable conductivity. And in bulk it is black. So even when your copper has a dark brown patina, you are actually looking through an extremely thin layer of the black oxide, at the copper underneath. Even a dark brown copper oxide layer is only about 40 nm thick, so even though the oxide may have 50 000 to 500 000 times the resistivity of copper, it only adds a resistance equivalent to a layer of copper 2 mm to 20 mm thick.

But what your rhetorical question may be missing is that the liquid "cooling" in the Tesla packs is not primarily there to reduce temperature. It is there to equalise temperature. This is so that parallel cells will continue to share current equally.

When temperature goes up, internal resistance goes down (at least in the short term). So when you have many cells in parallel, as Tesla do, if one of them gets just a little hotter than the others, maybe it's in the middle of the group, then its resistance will go down and it will carry even more current, leading to it getting even hotter... A positive feedback with negative consequences.

So the liquid coolant's primary job is to ensure that all the cells in a parallel group stay at the same temperature.

Another thing that manufacturers like Tesla can do, that is difficult for us small-scale builders, is to match cells so that those in a parallel group all have the same internal resistance and capacity. Actually, they don't need to be matched in both of those quantities. They only need to be matched in their product of internal resistance times capacity -- which has the units of "volt hours".

As well as matching internal resistance, you need to match external resistance. Actually, once again it is really external voltage drop that you need to match. One link may carry twice the current of another and so may need to have half the resistance.

It is easy to match external voltage drops with only two cells in parallel, by using "diagonal takeoff". i.e. their series connections to the other cell-pairs should always use the negative terminal of one cell and the positive terminal of the other. But any more than 2 cells in parallel is difficult.

[Edit: Removed some unnecessary hyperbole, and some uncertain reasons for the variability of copper(II) oxide conductivity.]

Last edited by weber on Tue, 27 Jun 2017, 11:22, edited 1 time in total.

One of the fathers of MeXy the electric MX-5, along with Coulomb and Newton (Jeff Owen).

You can see in those graphs, the Arrhenius-law aging with temperature -- approx doubling of the aging rate for every 10 °C rise -- no surprises there.

weber wrote: so long as you're not cycling them deeply every day, then temperature is the number one killer, and there's an exponential relationship between aging and temperature.

Are you sure they are not confusing with self discharge with actual capacity loss?
It appears as if they are comparing the voltage at different temps over time with a the first graph of a discharge curve.

Did they really loose capacity?
Where was the test at the end to cycle it to show the end capacity loss?

The "capacity fade" at 25DegC looks like the self discharge rate NOT capacity loss.

I'm not convinced.

Help prevent road rage - get outta my way! Blasphemy is a swear word. Magnetic North is a south Pole.

Hi Richo. I'd appreciate if you would quote my sentence in full, or at least begin your fragment with ellipses, since you've omitted an important proviso.

The periodic measurement of capacity by full charge and full discharge is described in some detail on the second page of the paper. Some of the authors may be students, but at least the first and last authors are staff of the Institute for Electrical Energy Storage Technology, Technical University of Munich. The first has the equivalent of our Master of Engineering degree, and the last, a Professor, has a PhD and is the head of the institute. I suggest that being published in a peer-reviewed journal is more important than "getting an A".

Last edited by weber on Tue, 27 Jun 2017, 11:55, edited 1 time in total.

One of the fathers of MeXy the electric MX-5, along with Coulomb and Newton (Jeff Owen).

ok, ok, ok, ok.
So they are actually measuring actual capacity and not loss of charge.
Well then I still find the results alarming
Some of them are showing 20% loss in under a year for a fully charged batt at high temp.

I was kind of hoping that it was an old paper and that they had low quality batteries to start with.
But this is quite recent and unless they got grey market batteries that counts the battery quality out too.

Damn!

It is interesting that the best ones were the ones left flat.
Although I'd still recommend something keeping it at it's minimum voltage.

As a matter of interest I purchased some of the crappiest 18650's I could find for cycle testing.
I just wanted to see how bad they really are.
The question being "I know they are crap but do I still get value for money?"
So I'll measure the Voltage, current and temp over time just on a continuous charge, discharge cycle.
I'll discharge one to destruction for peak power.

Any other requests?

No I wont write a white paper on it

Help prevent road rage - get outta my way! Blasphemy is a swear word. Magnetic North is a south Pole.

Richo - sling me a couple and I'll put them on my Arduino cycle tester. Always keen to know what the cycle life is. The more informative test is to do very slow discharges - this gives plenty of time for the competing reactions to take place and impact the Coulombic efficiency. But C/24 or even C/200 tests take for ever.

I think we have known for some time that high SOC and high temperatures are a Lithium cell killer. I know we have given advice to that extent (here) and I try to keep my SOC low when I can't help but leave the car in the hot sun - or leave it for any more than a few hours.

I am in trouble every now and then with the family when I complain that I wasn't warned that I'd need my car as I'd left it at 30% SOC or so.

Johny wrote:I am in trouble every now and then with the family when I complain that I wasn't warned that I'd need my car as I'd left it at 30% SOC or so.

So now you can see that for LFP there's no difference in ageing rates between 40% and 70% SoC, so you might as well charge it to 70% and not be in trouble so often. Apparently it's really about the voltage, not the SoC as such.

One of the fathers of MeXy the electric MX-5, along with Coulomb and Newton (Jeff Owen).

Yep, in Willetton now. I can charge at up to 10 amps, discharge at up to 30 amps. But rapid cycling only tells you so much - you really need to test Coulombic efficiency and to do this accurately you discharge slowly.

jonescg wrote: Richo - sling me a couple and I'll put them on my Arduino cycle tester. Always keen to know what the cycle life is. The more informative test is to do very slow discharges - this gives plenty of time for the competing reactions to take place and impact the Coulombic efficiency. But C/24 or even C/200 tests take for ever.

No that would be a C/∞ test. C/24 and C/200 tests take 24 and 200 hours.

To be clear: You're not claiming that your Arduino cycle tester can measure and control charge and discharge currents to the 5-digit (16-bit) precision required to usefully measure coulombic efficiency. Right?

Last edited by weber on Fri, 30 Jun 2017, 06:42, edited 1 time in total.

One of the fathers of MeXy the electric MX-5, along with Coulomb and Newton (Jeff Owen).

No, but if you want a robot to charge and discharge a battery hundreds of times, the equipment works very well. I cannot measure Coulombic efficiency with any of the equipment I have, despite this being a more useful thing to know than outright cycle number.

No, although the equipment is in there (shunts etc). I just cycle it 50 times, to a manual discharge and plot the results, then put it on for another 50 cycles. Rinse repeat. No point collecting cycles any more frequent than that really.